Frames comprised of reinforced concrete columns and girders for constructing buildings are well known. Such contemporary columns and girders are commonly constructed by first forming a latticework of rebar, i.e., a cage, which reinforces and contains the concrete. The cage, generally defining the column or girder, is surrounded by a form, commonly constructed of steel or fiberglass. Concrete is then poured into the form such that the cage is encapsulated thereby. The concrete is then typically vibrated to remove any voids formed therein. The form may be constructed in place such that the resulting column or girder need not be moved after the concrete cures. Alternatively, the form may be constructed at a convenient location, and the column or girder thus fabricated subsequently moved to its final location.
In multi-level commercial buildings, the steel latticeworks or cages for such columns and girders are commonly constructed by first disposing a plurality of elongate members or rebar upon a series of supports or horses and then positioning a plurality of sections of smaller diameter rebar or wire formed into generally rectangular hoops about the larger elongate rebar members to generally define the desired cage. Further elongate members may then be charged through these rectangular hoops and secured in position via wire ties.
As can be appreciated, this process is extremely labor intensive. Additionally, very loose tolerances, typically approximately 1/2 inch, are maintained due to the difficult nature of handling and aligning such materials. Thus, the lateral position of an elongate rebar member at the intersection of one rectangular hoop may vary by as much as 1/2 inch relative to its position at the intersection of another rectangular hoop. Such large tolerances are not desired. They are tolerated by building codes because of the present-day method of preforming the hoops and hooked cross-ties.
Typically such columns and girders are formed in thirty foot lengths, which are commonly required in building construction. Splice bars are shorter lengths, typically approximately sixteen feet, of rebar which are wire tied to the abutting ends of adjacent columns such that they may be joined thereby. As can be appreciated, such splicing greatly increases material usage, weight, and cost as well as requires substantial labor in the practice thereof. Column bars are spliced by overlapping their offset ends. Girder bars are usually just capped.
The need for frame structures to exhibit a comparatively high degree of ductility is particularly important in geographic locations known to experience substantial seismic activity. In such geographic locations it is not uncommon for frame structures to experience sufficient force to cause crushing or brittle failure of the concrete during seismic activity. Such crushing or brittle failure may result in catastrophic failure of the structural member.
For example, a portion of the encapsulating concrete may break away as a result of seismic activity. The breaking away of such a portion of the encapsulating concrete may then expose a portion of the rebar latticework or cage, allowing it to degrade from environmental factors, i.e. moisture, smog, etc., and also allowing it to move outward due to the lack of a retaining effect provided by the encapsulating concrete.
Furthermore, rectangular hoops are subject to rupture or breakage upon experiencing substantial seismic forces. Such substantial seismic forces may urge the rebar restrained by the rectangular hoop outward with sufficient force to pull apart the bent ends of the rectangular hoop. Columns using cross-ties with 90-degree bends, when subjected to bending and axial forces, have exhibited brittle failures caused by the 90-degree bends straightening out. Also intermediate longitudinal bars between cross-ties buckle outward due to lack of positive confinement, thus causing a brittle failure of the concrete. Thus, such construction is inadequate for use in geographic locations known to experience substantial seismic activity.
The prior art construction methods are thus labor intensive, require excessively large tolerances, utilize 90-degree bends which are failure prone, and additionally utilize intermediate bars which tend to buckle prematurely.
As such, although the prior art has recognized, to a limited extent, the problem of fabricating structural members such as columns and girders in a manner which will withstand substantial seismic forces, the proposed solutions have to date been in effective in providing a satisfactory remedy.